The present invention relates to nanomagnetic compositions and the use thereof in magnetic compositions, articles, and processes. More particularly, the present invention relates to nanomagnetic compositions and to processes for making and using in, for example, nanocompass and navigational articles and devices, alternating current transformers, and related applications. The present invention provides magnetic compositions comprising rotationally free, single domain, nanomagnetic composites.
The compositions of the present invention are useful in a variety of magnetically responsive systems, applications, and devices including, for example, as electrical transformer compositions and devices particularly at high electric field frequencies, magnetic gyroscopy and related navigational applications, for example, two and three dimensional compasses, optical communications, switching devices and systems, electromagnetic radiation generators and sensors, transducers, electromotive force (EMF) shielding applications, and the like.
The present invention also relates to processes for preparing magnetic compositions having substantially only rotationally free, single domain magnetic particles. More particularly, the present invention relates to magnetic compositions possessing novel magnetic properties including: being free of barriers to a change in system magnetization (M); having substantially no magnetic memory or hysteresis; having high initial magnetic permeability; wherein the magnetic response of the composition becomes infinite up to about the saturation magnetization of the composition upon application of a magnetic field (H); temperature and magnetic field dependent magnetic properties; solid-liquid phase dependent reversible superparamagnetic to paramagnetic properties; low magnetic resistivity; electrically insulating; and exceptionally high resistance to quantum tunneling above about 0.degree. K.
The term "domain" is described, for example, in C. P. Bean and J. D. Livingston, J. Appl. Physics, 30, 120 (1959); and B. D. Cullity, Introduction to Magnetic Materials, Addison-Wesley Publishing Co., MA, (1972), see also The Magnetic Properties of Materials, by J. E. Thompson, Newnes International Monographs on Materials Science and Technology, CRC Press, Cleveland, Ohio, 1968, which are incorporated herein by reference in their entirety, and refers in embodiments of the present invention to single domain particles, for example, discrete magnetically isolated and non interacting superparamagnetic nanoparticles. Although not wanting to be limited by theory, it is believed that the presence of substantially or exclusively single-domain crystallites in the compositions of the present invention enable the aforementioned combination of novel magnetic properties and applications thereof.
The present invention also relates to methods for forming and manipulating the magnetic properties of isolated nanocompass particles, for example, the size or dimensions of the single domain may be controlled to a great extent by the strength and duration of an externally applied pulsed electromagnetic field that is used to "carve" or etch individual domains, interstices or cavities within a continuous solid or gel phase.
A magnetic nanocompass composition and method as used herein refers to measuring the attitude and position of the magnetic composition before and after orienting the magnetic moments of the magnetic single-domain crystallites or nanoparticles and thereafter determining the relative magnetic changes with respect to reference coordinates thereby providing a three-dimensional compass composition and compass means as illustrated herein.
Properties of magnetic fluids are disclosed, for example, in Magnetic Fluids Guidebook: Properties and Applications, V. E. Fertman, Hemisphere Publishing Corp., N.Y., 1990, the disclosure of which is incorporated herein by reference in its entirety. The magnetic properties of magnetic fluids as a function of temperature have been extensively studied. For example, for conventional ferromagnetic or ferrimagnetic single-domain particles spontaneous magnetization disappears at a certain temperature known as the Curie point(T.sub.c), also known as the magnetic transition temperature. At the Curie point, the exchange atom interaction energy is equal to the thermal interaction energy, and the substance becomes paramagnetic. The upper Curie point is the temperature above which ferroelectric materials lose their polarization and the lower Curie point is the temperature below which some ferroelectric materials lose their polarization. When ferromagnetic materials become paramagnetic at the Curie point the material exhibits the so-called Curie-Weiss effect or behavior.
The following United States patents are noted as being of interest to the background of the present invention.
U.S. Pat. No. 5,316,699, issued May 31, 1994, to Ritter, Shull, et al., discloses a chemical process for producing bulk quantities of an iron-silica gel composite in which particle size, form, and magnetic state of the iron can be selected. The process involves polymerizing an ethanolic solution of tetraethylorthosilcate, with ferric nitrate present in water at low temperature under the influence of an HF catalyst. The chemical and magnetic states of the iron in the resultant composite are modified in situ by exposure to suitable oxidizing or reducing agent at temperatures under 400.degree. C. Iron-containing particles of less than 200 Angstroms diameter, homogeneously dispersed in silica matrices may be prepared in paramagnetic, superparamagetic, ferrimagnetic and ferromagnetic states.
U.S. Pat. No. 4,238,558, discloses low density magnetic polymeric carrier materials containing a polymer material impregnated with a magnetic elemental metal or metal oxide derived from transition metal carbonyl compounds. According to the disclosure of this patent, the carrier particles are prepared by placing in a suitable vessel particles of a polymeric material, a suspending medium, and a transition metal carbonyl, heating the mixture with agitation for the purpose of thermally decomposing the transition metal carbonyl, causing the polymer to be impregnated with a magnetic elemental metal or metal oxide of a transition metal carbonyl, followed by cooling.
The disclosures of each of the aforementioned documents are totally incorporated herein by reference.
U.S. Pat. No. 5,358,659 (D/91332) assigned to the assignee of the present application, and which is incorporated herein by reference in its entirety, discloses a method of forming magnetic materials having tunable magnetic properties and the magnetic materials formed thereby. The magnetic materials contain both single-domain and multi-domain particles and have high initial permeability while maintaining coercivity and remanence in the material. A method for making a magnetic ferrofluid comprises providing a colloidal suspension of submicron ion exchange resin matrix, loading the resin matrix by ultrafiltration with a magnetic ion, precipitating single-domain particles within said resin and precipitating multidomain particles outside of the resin to form a stable colloidal dispersion of the resin and particles.
In the aforementioned commonly assigned U.S. Pat. No. 5,362,417 (D/90063) there is disclosed a method of forming a colloidal dispersion of submicron particles comprising: providing an ion exchange resin matrix; loading said resin matrix with an ion; and treating the resin to cause in-situ formation of submicron particles; and fluidizing said ion exchange resin and particles in an aqueous medium to form a stable colloid of the particles.
U.S. Pat. No. 4,474,866, assigned to the assignee of the present application, discloses a developer composition containing superparamagnetic polymers. The developer composition disclosed in this patent consists of a dispersion of fine particles of iron oxide in a polystyrene ion exchange resin. More specifically, the developer composition consists of .gamma.--Fe.sub.2 O.sub.3 (gamma) disposed in a sulfonated divinylbenzene cross-linked polystyrene resin.
In the aforementioned commonly assigned copending application U.S. Ser. No. 08/332,174 (D/94178) is disclosed a method for producing a magnetized pigment comprising the steps of: forming a magnetic material core in a vaporized state from a vaporized magnetic material; and forming a pigment coating on the magnetic material core while in the vaporized state.
The disclosures of each of the aforementioned commonly assigned documents are totally incorporated herein by reference.
There exists a need for nanomagnetic compositions, articles, devices, and systems that are suitable for use at temperatures, for example, from about 100.degree. K to about 300.degree. K and above.
There remains a need for nanomagnetic materials which can be used in microscopic magnetic processes and applications. There also remains a need for magnetic materials which are single domain and rotationally free at temperatures of about 10.degree. K and above.
Still further, there is a need for nanocrystalline nanocomposite particles that permit low cost, clean, and optionally dry micron and submicron polymeric composite particles that can be selected for use in a magnetic liquid or solid formulation, and utilized as an active component in magnetic fluids, gels and solids.
Another problem in the field of magnetic materials has been the absence of rotationally free, single domain, superparamagnetic and paramagnetic compositions and processes for making and using.
Solutions to the above problems and needs have been unexpectedly found in the compositions and processes of the present invention wherein there is provided superior magnetic materials that enable, for example, nanomagnetic articles, devices and processes, for example, wherein the individual magnetic moments contained in the nanomagnetic crystalline species are freely rotating, single domain, and therefore easily and independently oriented in low, intermediate, and high magnetic fields.
A long standing problem in the area of nanomaterials and nanotechnology has been the absence of nanoscale magnetic materials which possess the necessary combination of magnetic properties to be suitable for use in, for example, navigational, and the like devices. In embodiments of the present invention, solutions to the aforementioned problem are provided.